Sawmill production lines typically employ rotating press roller assemblies to convey lumber at a controlled rate through saws, planers or other wood processing equipment. The rollers rotate about shafts which are connected to sprockets housed within a gearbox. Rotation of the sprockets and hence the roller shafts is driven by a long serpentine drive chain usually powered by a single electric motor. This type of mechanical drive system requires relatively costly maintenance. For example, the drive chain and sprocket system must be maintained in an oil bath for optimum performance.
Some planer mill infeed and outfeed rolls may alternatively be driven by hydraulic power to allow for variable frequency operation. Variable frequency operation is desirable, for example, for optimum processing of lumber pieces of different sizes (smaller pieces may require less planing and may be fed through the mill at a faster rate). Various hydraulic drive systems are in use. According to one existing system, a relatively small hydraulic motor is mounted on each roll shaft and is driven by a remote hydraulic pump. A torque arm is provided for counteracting rotation of the roller shafts. Such hydraulic drive systems are also relatively expensive to maintain and may result in environmental contamination and increased fire danger due to oil leaks from hydraulic pumps, motors, hoses, fittings or the like.
One strategy for overcoming the shortcomings of conventional mechanical and hydraulic drive systems is to couple each roller shaft directly to an electric motor. However, during normal operation planer mill roller shafts are deflected up and down depending upon the size and position of lumber passing through the mill. Previous attempts to mount electric motors directly on the ends of roller drive shafts have failed due to the weight of the motors and mechanical problems arising from periodic roller deflection.
The need has therefore arisen for a method and apparatus for economically converting a planer mill drive system from conventional mechanical or hydraulic power to variable frequency electric power.